Laser excitation of transversal and longitudinal polar modes in lithium niobate and tantalate crystals

Abstract

The excitation of longitudinal and transversal electromagnetic waves in lithium niobate and tantalate crystals is of interest for establish the conditions of coherent longitudinal and transversal waves generation in media and in vacuum. In this paper the results of laser excitation of transversal and longitudinal polar modes in these crystals are presented. We have measured spontaneous Raman spectra of lithium niobate and tantalate crystals in 0° (forward), 90°, and 180° (backward) scattering geometries. We have observed Raman peaks, related to fundamental transversal and longitudinal A1(Z) and E((X,Y) polar optical modes. In addition, there were pseudoscalar symmetry A2 peaks, forbidden by selection rules in Raman spectra for point group C3v. This was explained by reducing of the point group from C3v to C3 due to the presence of impurities in real crystals. Besides, the acoustic biphonon at low frequency has been observed. High intensity of spontaneous A1(Z)LO and A1(Z)TO Raman satellites gives the opportunity for generation of coherent longitudinal and transversal terahertz waves in lithium niobate and tantalate crystals with the help of Stimulated Raman Scattering under using high-power laser pumping. The presence of pseudoscalar and biphonons mode in low frequency region results in the strong interaction with fundamental soft mode and sharp central peak near the phase transition.